Chapter 26: Q34P (page 1343)

Plywood and particle board are often glued with cheap, waterproof urea-formaldehyde resins. Two to three moles of formaldehyde are mixed with one mole of urea and a little ammonia as a basic catalyst. The reaction is allowed to proceed until the mixture becomes syrupy, and then it is applied to the wood surface. The wood surfaces are held together under heat and pressure, while polymerization continues and cross-linking takes place. Propose a mechanism for the base-catalyzed condensation of urea with formaldehyde to give a linear polymer, and then show how further condensation leads to cross-linking. (Hint: The carbonyl group lends acidity to the N - Hprotons of urea. A first condensation with formaldehyde leads to an imine, which is weakly electrophilic and reacts with another deprotonated urea.)

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Condensation polymerization

In this process, a large number of monomer molecules combine together usually with the loss of simple molecules like water, alcohol, ammonia, carbon dioxide, hydrogen chloride etc., to form a polymer in which the molecular formula of the repeat unit is not the same as that of the monomer.

Urea-formaldehyde resin

This type of polymer is obtained by a reaction between urea and formaldehyde in the presence of a base or an acid.

Mechanism and further condensation

Ammonia (NH₃) acts as the basic catalyst.

mechanism for the base-catalyzed condensation of urea with formaldehyde

The linear molecules can condense with more formaldehyde molecules to form cross-linked structures. Also, there is a possibility for dimethyl urea to condense with each other to form linear polymers.